1. Field of the Invention
This invention relates to a method of and an apparatus for vacuum casting in which upon reaching of a predetermined pressure reduction degree of a cavity formed in a die, a sprue is opened by a gate member, causing the molten metal collected in a molten metal passage and a molten metal reservoir to flow into the cavity for casting.
2. Description of the Prior Art
FIG. 10 shows a related art apparatus which has been proposed by the applicant (Japanese Patent Application No. 4-309534). The Japanese application is not laid open on the priority date of the present application.
As shown in FIG. 10, in the related art vacuum casting process, when a die 50 having been closed is set over a gas-tight furnace 59, the pressure in the gas-tight furnace 59 is increased by a pressurizing means (not shown), so that the molten metal stored in a molten metal storage tank 59r is pushed up to the interior of a molten metal reservoir 52 through a molten metal passage 58. When a predetermined level is reached by the molten metal, the pressure in the gas-tight furnace 59 is held at a constant value. Further, substantially simultaneously with the pressure increase in the gas-tight furnace 59, the pressure in a cavity 56 is reduced by an evacuating pump (not shown). When a predetermined pressure reduction degree is reached in the cavity 56, a gate member 52a is raised to open a sprue 56a, thus causing the molten metal having been collected in the molten metal passage 58 and the molten metal reservoir 52 to flow into the cavity 56.
In the above vacuum casting process, however, the interior of the molten metal reservoir 52 is open to atmosphere. Therefore, the head in the molten metal reservoir 52 is greatly reduced during the flow of the molten metal into the cavity 56. This is caused by a pressure loss due to movement of the molten metal while the differential pressure between the pressure in the gas-tight furnace 59 and the pressure in the molten metal reservoir 52 is constant. In order to prevent air in the molten metal reservoir 52 from being withdrawn into the cavity 56, it is necessary to set the head in the molten metal reservoir 52 to a somewhat high level. That is, it is necessary to collect a great quantity of molten metal in the molten metal reservoir 52. Doing so poses the problems of elongation of the cycle time of the casting and great reduction of the molten metal temperature before the casting. Further, the molten metal reservoir 52 should have a large height, thus posing a problem of size increase of the casting apparatus.
A technique in which molten metal to be introduced into the cavity is led to a molten metal reservoir in advance, is disclosed in Japanese Laid-Open Patent Publication No. 3-198969. This technique is illustrated in FIG. 11. In this instance, a sprue 103a of a cavity 103 is opened and closed by a gate member 109. A molten metal passage 113 is communicated with a molten metal storage tank 107. The cavity 103 and the molten metal passage 113 are communicated with each other via the gate member 109 and a molten metal reservoir 119. The molten metal reservoir 119 has an upwardly extending branch 115, the top of which is communicated with pressure reducing means 117.
In this system, with the sprue 103a closed by the gate member 109, the pressure in the molten metal reservoir 119 and the branch 115 is reduced by the pressure reducing means 117 to lead the molten metal in the molten metal storage tank 107 through the molten metal-passage 113 to the molten metal reservoir 119 and the branch 115. Simultaneously with the pressure reduction by the pressure reducing means 117, the pressure in the cavity 103 is reduced by a vacuum pump 105. When the pressure in the cavity 103 is reduced to a predetermined pressure, the gate member 109 is pulled up to communicate the cavity 103 with the molten metal reservoir 119 and the branch 115, so that the molten metal having been led to the molten metal reservoir 119 and the branch 115 is led into the cavity 103.
In this case, the pressure reduction in the molten metal reservoir 119 and the branch 115 is continued by the pressure reducing means 117 while the gate member 109 is pulled up. Thus, the molten metal head in the molten metal reservoir 115 is not greatly reduced while molten metal is introduced into the cavity 103. Thus, the problem noted above can be solved to a considerable extent. However, molten metal is introduced into the molten metal reservoir 119 and the branch 115 with its lowest head (in this case the head in the molten metal reservoir 119 and the head right underneath the gate member 109 being lower than the head in the branch 115) lower than the level of the sprue 103a. Therefore, gas and/or foreign particles floating on the low head surface are liable to be withdrawn into the cavity 103.